1. Field of the Invention
The present invention relates to a process for crystallizing fructose and a device therefor. More particularly, it relates to a process for continuously recovering large and uniform crystals of fructose in a good yield from an aqueous solution thereof and for facilitating separation of syrup from the crystal.
2. Description of the Prior art
Generally speaking, crystallization of fructose must be carried out from an aqueous fructose solution of a high concentration because fructose has a high solubility in water. However, it is quite difficult to separate syrup from crystalline massecuite because of the high viscosity of the aqueous solution caused by its high concentration. Further, fructose is highly soluble, as is apparent from its solubility of 87 W/W % at 50.degree. C., is liable to decompose at a high temperature or due to changes in pH, and is liable to polymerize. Consequently, more care must be taken in crystallizing fructose anhydride than in crystallizing sucrose or glucose.
Evaporation in a concentration device has been conventionally carried out at a low temperature under high vacuum because of the unstability of fructose. However, in batch concentration devices, a long concentration time and a high concentration temperature are needed owing to a large liquid depth leading to boiling point elevation, so that the fructose tends to decompose to bring about coloration or polymerization.
In recent years, accordingly, a continuous concentration system begins to be employed that needs only a short concentration time and a low concentration temperature in concentrating aqueous fructose solutions and aqueous fructose-glucose solutions. The employment of such a continuous concentration system for concentrating sugar solution has led inevitably to the demand for a continuous crystallization device.
Most of conventional cooling crystallization equipments generally used are of an internal cooling batch system in which cooling water is introduced into a horizontal jacket or an agitating part or ribbon mixer thereof. Conventional continuous systems are a mere combination of a number of the above batch systems. That is to say, the batch cooling crystallization equipment includes the steps of feeding raw material, crystallizing sugar and separating crystals, and needs at least three crystallizing devices. Since each of the crystallizing devices must be accompanied by attachment devices such as automatic instruments, the total cost of the equipment is markedly great.
The batch crystallizing operation is performed by adding 1 to 5% of seed crystals to at least 90 W/W % of concentrated fructure solution at 60.degree. to 65.degree. C. and gradually cooling the solution to grow the crystal particles, while the degree of supersaturation is controlled to a low value to inhibit the spontaneous generation of crystals. This operation inevitably needs a long crystallization time, and also needs a long time at the high temperature stage at the initial stage, so that denaturation such as coloration or polymerization due to the decomposition of the sugar may occur.
We, the inventors of the present invention have studied to overcome the above-mentioned drawbacks of the prior art and to realize the continuous crystallization of fructose anhydride from the aqueous solution thereof. As the result, we have succeeded in the development of a continuous process of the present invention for obtaining large and uniform fructose anhydride crystals with syrup separability in a good yield without any denaturation of sugars. According to a known process for crystallizing fructose anhydride, an aqueous solution containing about 95% of fructose is concentrated up to a concentration of 92 to 94 W/W % in solid content and placed in an auxiliary crystallizer, and the temperature of the solution is lowered carefully from a temperature of 60.degree. to 65.degree. C. to a temperature of 30.degree. to 35.degree. C. at a rate of 1.degree. C. per four hours after addition of 1 to 5% of powdered fructose as seed crystals. The yield is 40 to 50%. (see Shinji Tanaka, SWEETNING AGENT, published by Korin Shoin)
However, it was found from the analysis of products that the above process results in a weight loss of 2 to 10% of fructose and a marked increase in the amount of polymerized fructose owing to the decomposition and polymerization of fructose caused by holding the solution at a high temperature of 60.degree. to 65.degree. C. for a long time. There has been also proposed a process wherein the pH of the solution is controlled to within 4.5 to 5.5, particularly 5.0, by the use of sodium carbonate or the like to improve the yield because the formation of the polymerized fructose depends on the Ph of the solution (see Japanese Patent Publication No. 105842/1975).
In recent years, a very high quality is demanded also for the high-fructose solution as the amount of the fructose-glucose solution used for soft drinks increases. Therefore, the high-fructose solution is prepared by the use of purifying steps of decoloring with activated charcoal, desalting with ion exchange resins and the like. As the result, salts are scarcely contained in the fructose solution, and the solution has a quite small buffering action. For example, when a solution having a solid content of 91.3 W/W %, a pH of 4.9 and a fructose content of 96.8% was held at 60.degree. C. for 10 hours, the pH of the solution decreased to 3.8 and the fructose content decreased to 93.6%, i.e., a loss of 3.2%. When the same solution as mentioned above was held at 45.degree. C. for 10 hours, the pH decreased to 4.6 and the fructose content decreased to 96.7%.
As these examples show, if the decomposition and polymerization are controlled by regulating the pH of the solution, the weight loss of fructose of several % is inevitable because of the formation of acidic substances by the decomposition of fructose.